This invention relates generally to a system and method for producing metallic iron by thermally reducing a metal oxide in a moving hearth furnace.
Metallic iron has been produced by reducing iron oxide such as iron ores, iron pellets and other iron sources. Various such methods have been proposed so far for directly producing metallic iron from iron ores or iron oxide pellets by using reducing agents such as coal or other carbonaceous material.
These processes have been carried out in rotary hearth and linear hearth furnaces. An example of such a rotary hearth furnace is described in U.S. Pat. No. 3,443,931. An example of such a linear hearth furnace is described in U.S. Pat. No. 7,413,592. Both the rotary hearth furnace and the linear hearth furnace involve making mixtures of carbonaceous material with iron ore or other iron oxide fines into balls, briquettes or other compacts, and heating them on a moving hearth furnace to reduce the iron oxide to metallic iron nuggets and slag.
Hearth furnaces are largely operated with combustion gases from the heating burners flowing counter to the movement of the hearth and the charge materials. Thermal energy is transferred to the charge materials by direct radiation from the burner flame and the furnace walls as well as by direct contact of the combustion gases with the charge materials. The open nature of these systems, even if divided into zones by baffle walls, does not allow much control of the furnace atmosphere, which is predominantly burner combustion products with lesser amounts of reaction products from the charge materials.
Hearth furnaces are generally heated by natural gas burners that provide thermal energy to the system to raise the temperature of the charge materials and initiate the reduction process, that is, the reaction of the carbon in the charge materials with the iron oxides in the charge materials. The carbon dioxide in the combustion gases also reacts with the carbon in the charge materials to produce carbon monoxide through the Boudouard reaction at about 1830° F. (1000° C.). This reaction removes carbon from the charge materials at relatively low temperatures. At these temperatures the reaction rate between the carbon monoxide formed and the iron oxide is relatively slow. Therefore, carbon is leached from the system before the reduction process can be completed and has a negative effect on the process.
The effect is that final reduction, in the case of forming iron nuggets, relies on production of carbon monoxide through interaction of the combustion gases with the carbon in the charge materials, which requires high temperatures approaching or exceeding 2550° F. (1400° C.). This high temperature requires both extra burner energy and time to allow completion of the reduction process. These higher temperatures also increase construction and maintenance costs because more costly refractory is required. A further impediment is the removal of carbon from the charge materials before metallization is complete so that the carbon is not available to be absorbed by the metallic iron formed reducing its melting temperature.
An additional limitation of these furnaces, and the methods of operating these furnaces, in the past has been their energy efficiency. Furthermore, the reduction process involved production of volatiles in the furnace that had to removed from the furnace and secondarily combusted to avoid an environmental hazard, which added to the energy needs to perform the iron reduction. See, e.g., U.S. Pat. No. 6,390,810. What has been needed is a furnace that reduces the energy consumption needed to reduce the iron oxide bearing material such that a large part, if not all, of the energy to heat the iron oxide bearing material to the temperature necessary to cause the iron oxide to be reduced to metallic iron and slag comes from combusting volatiles directly in the furnace itself and otherwise using heat generated in one part of the furnace in another part of the furnace.
A hearth furnace for producing metallic iron material is disclosed that comprises:
(a) a furnace housing having a drying/preheat zone capable of providing a drying/preheat atmosphere for reducible material, a conversion zone capable of providing a reducing atmosphere for reducible material, a fusion zone capable of providing an atmosphere to at least partially reduce metallic iron material, and optionally a cooling zone capable of providing a cooling atmosphere for reduced material containing metallic iron material, the conversion zone being positioned between the drying/preheat zone and the fusion zone,
(b) a hearth capable of being movable within the furnace housing in a direction through the drying/preheat zone, then the conversion zone, then the fusion zone, and then, if present, the cooling zone,
(c) a hood or separation barrier positioned within at least a portion of the conversion zone, fusion zone or both, separating the conversion and fusion zones where the hood or separation barrier is positioned into an upper region and a lower region with the lower region adjacent the hearth and the upper region adjacent the lower region and spaced from the hearth, and
(d) at least one reductant injector capable of introducing a gaseous reductant into the lower region adjacent the hearth.
Alternatively, the hood or separation barrier may be positioned within at least a portion of the conversion zone, the fusion zone or both, separating the furnace housing where the hood or separation barrier is located into a combustion region and a reducing region with the reducing region adjacent the hearth and the combustion region adjacent the reducing region and spaced from the hearth. In some alternatives, the separation barrier may act as a hood.
In addition, a method of reducing iron ore and other iron oxide sources is disclosed comprised of:
(a) providing a furnace housing having a drying/preheat zone capable of providing a drying/preheat atmosphere for reducible material, a conversion zone capable of providing a reducing atmosphere for reducible material, a fusion zone capable of providing an atmosphere to at least partially reduced metallic iron material, and a cooling zone capable of providing a cooling atmosphere for reduced material containing metallic iron material, the conversion zone being positioned between the drying/preheat zone and the fusion zone,
(b) providing a hearth capable of being movable within the furnace housing in a direction through the drying/preheat zone, then the conversion zone, then the fusion zone, and then the cooling zone,
(c) positioning a hood or separation barrier within at least a portion of the conversion zone, fusion zone or both separating the atmosphere of the conversion and fusion zones where the hood or separation barrier is positioned into an upper region and a lower region, with the lower region adjacent the hearth and the upper region adjacent the lower region and spaced from the hearth,
(d) injecting a gaseous reductant into the lower region adjacent the hearth, and
(e) moving the hearth containing iron oxide bearing material and carbonaceous material in the furnace housing through the drying/preheat zone to dry and preheat the iron oxide bearing material and carbonaceous material, then through the conversion zone to heat the iron oxide bearing material and carbonaceous material to at least partially reduce the iron oxide bearing material, then through the lower region of the fusion zone in the presence of the injected gaseous reductant to fuse the reduced iron oxide bearing material to metallic iron material, and then through the cooling zone to cool the metallic iron material.
The gaseous reductant may be selected from the group comprising carbon monoxide, hydrogen, natural gas, syn-gas, or mixtures thereof.